1. Field of the Invention
The present invention relates to a CDMA (Code Division Multiple Access) mobile communication system and method using a spread spectrum communication system. Further, the present invention is concerned with a spread spectrum communication transmitter and receiver used for such a CDMA mobile communication system.
2. Description of the Related Art
FIG. 1 is a block diagram of a base station transmitter used in a CDMA mobile communication system using a conventional spread spectrum communication system, which is typically described in the IS/95 that is a standard system in the U.S. Telecommunications Industry Association/Electronic Industries Association (TIA/EIA). FIG. 2 is a block diagram of a mobile station receiver in the CDMA mobile communication system.
The transmitter shown in FIG. 1 can simultaneously communicate with n mobile stations where n is an integer. More particularly, the transmitter includes traffic channel transmit units 311, 312, . . . , and 31n, which respectively communicate with the first, second, . . . , and nth mobile stations. Each of the traffic channel transmit units 311 through 31n includes an information modulator 2 and a spread spectrum modulator 5. The information modulator 2 of each traffic channel modulates transmit data (information) 4 by a BPSK, QPSK or another modulation method. The modulated transmit data is applied to the spread spectrum modulator 5. The spread spectrum modulators 5 of the traffic channel transmit units 311 through 31n generate respective spreading codes (PN codes). The spread spectrum modulator 5 of each traffic channel spread the spectrum of the modulated transmit data from the information modulator 2.
The transmitter shown in FIG. 1 has a pilot channel transmit unit 30. The mobile receivers discriminate the base stations from each other by referring to the pilot channel. The pilot channel transmit unit 30 includes a pilot data generator 1, an information modulator 2 and a spread spectrum modulator 3. The information modulator 2 modulates pilot data generated by the pilot data generator 1 by the BPSK, QPSK or another modulation method. The spread spectrum modulator 3 spreads the spectrum of the modulated pilot data by using a spreading code specifically used for the pilot channel and different from the spreading codes used for the traffic channels. The pilot signal thus generated can be arbitrary data which can be known in the base stations and the mobile receivers. For example, data consisting of only binary ones or binary zeros can be used as the pilot data.
The output signals of the traffic channel transmit units 311 through 31n and the pilot channel transmit unit 30 are combined so that the pilot channel and the traffic channels are simultaneously transmitted in a given frequency band. Then, the combined radio signal is transmitted via an antenna.
FIG. 3 shows a relation between the pilot and traffic channels with respect to time. As shown in FIG. 3, the pilot signal is always transmitted without any interval. In this regard, the pilot signal is a continuous signal.
Referring to FIG. 2, the mobile receiver used in the conventional CDMA mobile communication system includes a pilot channel receive unit 34, and a traffic channel receive unit 35. The pilot channel receive unit 34 includes a despreader 8, a path detector 11 and a hand-over controller 19. The traffic channel receive unit 35 includes despreaders 9 and 10, a RAKE combiner 12, an information demodulator 13, and a level measuring unit 14 for controlling a transmit power.
The despreader 8 performs a despreading process on the received signal by using the spreading code for the pilot channel. The despreaders 9 and 10 perform a despreading process on the received signal by using the spreading code allocated to the receiver shown in FIG. 2 at the transmitter. The path detector 11 detects multiple paths from the pilot signal. The hand-over controller 19 performs a hand-over control by using the results of the multipath detection obtained by the path detector 11. The output signal of the path detector 11 is also used as a timing signal used for the despreading process carried out by the despreaders 9 and 10. The RAKE combiner 12 performs a RAKE process on the despread signals from the despreaders 9 and 10. The information demodulator 13 demodulates the output signal of the RAKE combiner 12 to thereby generate the original information. The level measuring unit 14 performs a level measuring operation for controlling the transmit power.
FIG. 4 shows a cell structure of the CDMA mobile communication system having the above transmitter and receiver. There are illustrated first, second, third and fourth base stations 21, 22, 23 and 24, which cover service areas (cells) 26, 27, 28 and 29, respectively. All the base stations 21 through 24 have transmitters as shown in FIG. 1. A reference number 25 indicates a mobile receiver (station) having the structure shown in FIG. 2. The mobile station 25 is located within the cell 26 covered by the base station 21, and can communicate with the base station 21.
FIG. 5 is a timing chart of timings at which the base stations 21 through 24 respectively transmit the pilot signal. In the conventional CDMA mobile communication system, all the base stations 21 through 24 employ the same spreading code for spreading the pilot data. The period of the spreading code used to spread the pilot data is sufficiently longer than one symbol time of information (data). As shown in FIG. 5, the base stations 21 through 24 transmit the same spreading code for the pilot channel with respective inherent offset times equal to a time t′. That is, the starting points of the spreading codes used in the base stations 21 through 25 are offset by the time t′.
The mobile station 25 shown in FIG. 4 receives the pilot signals from the base stations 21, 22, 23 and 24. Usually, the pilot signal from the base stations 21 closet to the mobile station 25 has the strongest level. The despreader 8 of the pilot channel receive unit 34 shown in FIG. 2 performs the despreading process on the received signal by using the same spreading code as used in the transmitter.
FIG. 6A shows a correlation between the spreading code for the pilot channel and the pilot signal transmitted by the base station 21 and received by the mobile station 25. Similarly, FIGS. 6B, 6C and 6D show correlations with the pilot signals transmitted by the base stations 22, 23 and 24 and received by the mobile station 25. Peaks 201 through 204 respectively shown in FIGS. 6A through 6D indicate timing synchronization points in the pilot channels of the base stations 21 through 24. Variations in the waveforms other than the peaks 201 through 204 shown in FIGS. 6A through 6D result from a self-correlation of the spreading code for the pilot channel. These variations in the waveforms are noise components for the mobile station 25 (receiver).
The mobile station 25 shown in FIG. 4 receives the signals of the pilot channels transmitted by the base stations 21 through 24 in such a state that the signals are superimposed. Hence, the output signal of the despreader 8 of the pilot channel receive unit 34 has a formation in which the four waveforms shown in FIGS. 6A through 6D are superimposed. It should be noted that the correlations shown in FIGS. 6A through 6D are not affected by multipath fading or Rayleigh fading.
The path detector 11 shown in FIG. 2 detects the greatest peak in the output signal of the despreader 8 (the greatest peak in the superimposed correlation waveform). In the case of FIG. 4, the mobile station 25 is located within the cell 26 of the base station 21. Hence, the propagation distance between the base station 21 and the mobile station 25 is shorter than the propagation distances from the base stations 22, 23 and 24. Hence, the path between the base station 21 and the mobile station 25 has the smallest propagation loss. Hence, the greatest peak in the despread received signal output by the despreader 8 corresponds to the correlation peak 201 of the pilot channel of the base station 21 having the cell 26 in which the mobile station 25 is located.
Since the pilot signals transmitted by the base stations 21 through 24 have respective inherent time offsets. Hence, by detecting the greatest peak of the superimposed correlation waveform, it is possible for the mobile station 25 to discriminate the base station 21 from the other base stations 22 through 24 and detect the timing of spectrum-spreading. The path detector 11 informs the despreaders 9 and 10 of the traffic channel receive unit 35 of the timing of the greatest peak 201.
The despreaders 9 and 10 perform the despreading processes on the received signal of the allocated traffic channel at the timing informed by the path detector 11. The RAKE combiner 12 performs a RAKE combination process (path diversity combination) on the output signals of the despreaders 9 and 10 by using information concerning the pilot channel supplied from the path detector 11. The above information includes information concerning the timing, amplitude (receive power level) and phase of the pilot signal. The information demodulator 13 demodulates the output signal of the RAKE combiner to thereby produce the original information (data).
The level measuring unit 14 measures the received signal of the traffic channel by using the output signal of the RAKE combiner 12, and controls the transmission power of the mobile station 25. It will be noted that a transmit part of the mobile station shown in FIG. 2 is omitted for sake of convenience.
The hand-over controller 19 performs a control by using the output signal of the path detector 11 so that the mobile station 25 is handed over to the area of the base station which transmits the pilot signal received as the greatest peak at the mobile station 25.
However, the conventional CDMA mobile communication system thus configured has a disadvantage in that a good S/N ratio cannot be obtained at the time of receiving the pilot signals from the base stations due to the fact that all the base stations continue to transmit the pilot signals. The mobile station 25 shown receives the pilot signal from the base station 21 to which the mobile station 25 belongs so that the signals of the pilot channels transmitted by the other base stations 22, 23 and 24 are superimposed, as noise components, on the pilot channel data signal from the base station 21. Hence, the pilot channel receive unit 34 does not have a goon S/N ratio.
The signals of the pilot channels transmitted by the base stations 22 through 24 serve as interference signals with respect to the signal of the traffic channel processed by the traffic channel receive unit 35 of the mobile station 25. That is, the mobile station 25 always receives the signals of the pilot channels transmitted by the base stations 22 through 24 to which the mobile station 25 does not belong, and thus always receives interference by the base stations 22 through 24. Hence, the given frequency range can accommodate only a reduced number of stations (corresponding to a channel capacitance).